One of the unique things you can see while in Seattle is the Boeing Tour. Boeing will let you in to take a tour of their assembly plant, where you can see various planes -- 757, 777, 767, and maybe some others, getting worked on.
It may sound like something geeky, but seeing the huge planes up close, in pieces, and being worked on is amazing. You also learn some great trivia about how these wonderful machines are put together.
I've told friends that this tour is required if they come visit -- it's one of my favorite tours and definately worth the price.
more